Curriculum

"You have had five teachers in this subject so far, I believe. Naturally, these teachers will all have had their own methods and priorities. Given this confusion I am surprised so many of you scraped an O.W.L. in this subject. I shall be even more surprised if all of you manage to keep up with N.E.W.T. work, which will be much more advanced."

Behind the scenes

Tom Riddle twice applied for the Defence Against the Dark Arts position, the first time shortly after graduation from Hogwarts when he was turned down by HeadmasterArmando Dippet, who claimed he was too young but invited him to re-apply a few years later. Many years later, after becoming steeped in the Dark Arts and starting to use the title "Lord Voldemort", he visited the school and applied again, this time to Dumbledore. He was refused again. According to Albus Dumbledore, after the second refusal, no Defence Against the Dark Arts teacher held the post for more than one school year. Dumbledore believed that this was due to Voldemort placing a curse on the position, a believe that was popularly spread throughout the school.

When J. K. Rowling was asked for the reason Dumbledore would not give Severus Snape the Defence Against the Dark Arts job, the author responded that Dumbledore believed that teaching Defence Against the Dark Arts would bring out Snape's worst side, though Dumbledore finally relented in Harry's sixth year. [1]

J.K. Rowling has also revealed that after Amycus Carrow was the Dark Arts teacher, a permanent teacher took the post, because Voldemort's curse over the job was lifted.

Whilst it is known that Dumbledore did not wish Professor Snape to teach D.A.D.A. for fear of relapsing into his Death Eater ways it is also possible that (since the subject had been jinxed) he was also thinking of Snape's physical well-being (not least of all because of how useful he could have been, and was, in the long run).

Each Defence Against the Dark Arts teacher attacked (or attempted to attack) Harry. Quirrell tried to strangle Harry and jinxed his broom during a Quidditch game, Lockhart attempted to erase his memories, Lupin went after him as a werewolf (although he alone out of all the others would never have attacked Harry had he been in his human form), Barty Crouch Jr. (disguised as Moody) attempted to kill Harry, Umbridge physically assaulted him, and Snape attacked Harry in retaliation for the latter trying to curse him during the Battle of the Astronomy Tower. Ironically, Amycus Carrow, who taught Dark Arts instead of Defence Against the Dark Arts, was attacked by Harry Potter after the former had insulted Minerva McGonagall.

Dolores Umbridge was the only female Defence Against the Dark Arts teacher in the series, besides Galatea Merrythought (the latter of whom was not actually seen throughout the series, only mentioned).

The six professors of this subject who taught Harry each had a dark secret that he didn't learn until near the end of their respective school years:

Quirrell was serving Voldemort and was possessed by him.

Lockhart was a cowardly fraud who covertly stole the achievements of others.

Lupin was a werewolf.

Moody was actually Barty Crouch Jr., an escaped Death Eater.

Umbridge was the one who sent the Dementors that attacked Harry and Dudley.

Snape was the one who had reported the Prophecy to Voldemort, and was the Half-Blood Prince.

Most of the D.A.D.A. teachers in Harry's six years at Hogwarts followed a similar pattern: each (with the exception of Quirrell and Snape) was an entirely new teacher, each was chosen out of necessity, due to the loss of the previous instructor and the absence of other applicants, each meets Harry before he arrives at Hogwarts in the respective year (with the exception of Mad-Eye Moody, who was only mentioned in passing - although Harry had unknowingly encountered Barty Crouch Jr at the Quidditch World Cup), and each physically (or magically) attacks Harry during school term, either intentionally, or accidentally as in the case of Remus Lupin.

In Order of the Phoenix, when discussing who their new D.A.D.A. teacher would be, Harry said that the teachers didn't have much luck as "One sacked, one dead, one lost his memory and one was locked in a trunk for nine months", referring to Lupin, Quirrel, Lockhart and Moody respectively; this isn't true. However, as Lupin wasn't sacked. Rather, he resigned.